A method called an MS/MS analysis (or tandem analysis) is known as one of the mass spectrometric techniques for identification, structural analyses or quantitative determination of a substance having a large molecular weight. A triple quadrupole mass spectrometer is a typical example of MS/MS mass spectrometers.
FIG. 3 is a schematic configuration diagram of a commonly used triple quadrupole mass spectrometer. This triple quadrupole mass spectrometer is provided with a collision cell 14 including an ion guide 15 having four or more poles, with two quadrupole mass filters 13 and 18 for separating ions according to their mass-to-charge ratios m/z respectively provided on the front and rear sides of the collision cell 14. Among a variety of ions produced in an ion source 11, only a target ion having a specific mass-to-charge ratio is selected by the front-stage quadrupole mass filter 13 and introduced into the collision cell 14. The introduced ion collides with CID gas within the collision cell 14, to be dissociated into various kinds of product ions. Since this dissociation occurs in various forms, a plurality of kinds of product ions with different mass-to-charge ratios are normally produced from one kind of precursor ion. Those kinds of product ions are introduced into the rear-stage quadrupole mass filter 18, by which only an ion having a specific mass-to-charge ratio is selectively allowed to reach the detector 19.
The mass-to-charge ratio of an ion that can pass through the quadrupole mass filters 13 and 18 depends on the values of the radio-frequency voltage and the direct-current voltage applied to the rod electrodes constituting those mass filters 13 and 18. Accordingly, by continuously varying the mass-to-charge ratio of an ion that can pass through one of the front-stage and rear-stage quadrupole mass filters 13 and 18 while maintaining the mass-to-charge ratio of an ion that can pass through the other one of the quadrupole mass filters 13 and 18, it is possible to perform a precursor-ion scan for searching for every precursor ion from which a specific kind of product ion is produced, or conversely, a product-ion scan for searching for every product ion which is produced from a specific kind of precursor ion. A neutral-loss scan for searching for every precursor ion from which a specific kind of structural part is desorbed can also be performed, in which case the mass-to-charge ratios at which ions are allowed to pass through the two quadrupole mass filters 13 and 18 are continuously varied so that the two selected ions constantly maintain a specific difference in mass-to-charge ratio.
Mass spectrometers, including the triple quadrupole mass spectrometers, are often used as a detector for a gas chromatograph (GC) or liquid chromatograph (LC) for temporally separating various kinds of components in a sample. In the case of a GC/MS having a gas chromatograph (GC) combined with a mass spectrometer, the largest portion of the sample gas introduced into the ion source 11 of the mass spectrometer is the carrier gas used in the GC. As the carrier gas, a noble gas, such as helium (He), is generally used. In particular, when an electron ionization method is used, helium easily receives an amount of energy in the ion source and turns into a metastable atom (molecule). Metastable helium is hereinafter expressed as He*.
A He* molecule is electrically neutral but has a higher level of excitation energy than stable helium, He. Therefore, if a He* molecule is ejected from the ion source 11 and travels like an ion, the He* molecule interacts with various kinds of surrounding atoms or molecules, causing a self-ionization of He*, or conversely, a secondary ionization of the surrounding atoms or molecules. Ions produced by such processes constitute a major cause of the background noise and lower the signal-to-noise ratio. To reduce such a noise due to the He* molecules (or the atoms or molecules of other kinds of metastable noble gas), various configurations for the mass spectrometer have been conventionally proposed.
For example, in a mass spectrometer disclosed in Patent Document 1, a curved ion guide is used, in which target ions are made to travel along a curved ion-beam axis, while the electrically neutral He* molecules are made to travel straight and deviate from the ion-beam axis. Thus, He* molecules are prevented from entering the mass analyzer and the detector which are located on the rear side of the ion guide.
In the mass spectrometers disclosed in Patent Documents 2 and 3, a collision chamber into which N2 or similar inert gas is introduced is provided on the front side of the mass analyzer, and He* is passed through this chamber so as to make He* and N2 come in contact with each other and thereby ionize N while turning He* into stable helium, He. Thus, the metastable helium (He*) is prevented from entering the mass analyzer.
However, any of the previously described conventional techniques has a problem. That is to say, the ion guide for transporting ions normally consists of a plurality of multi-pole rods with four or more poles, and assembling curved multi-pole rods with high dimensional accuracy is considerably expensive. Furthermore, if its mechanical accuracy is inadequate, the passing efficiency of the target ion will be low, which leads to a decrease in the sensitivity.
In the case of removing He* by making it come in contact with N2 or similar gas, the target ion is also made to pass through the same gas area, so that the passing efficiency of the ions deteriorates and the signal level in the detector decreases. Therefore, the S/N ratio does not always improve even if the noise is reduced. Another problem is that converting He* into He requires creating an area with a considerably high density of N2 gas, which means that a high evacuation power is needed to maintain the neighboring vacuum chamber in a high-vacuum state.